Process for surface conditioning of a plate or sheet of stainless steel and application of a layer onto the surface, interconnect plate made by the process and use of the interconnect plate in fuel cell stacks

Abstract

A process for the conditioning of and applying a ceramic or other layer onto the surface of a sheet of stainless steel comprises the steps of (a) optionally annealing the steel plate or sheet in a protective gas atmosphere at an elevated temperature, (b) controlled etching of the surface of the sheet to produce a roughened surface and (c) depositing a protective and electrically conductive layer onto the roughened metallic surface. The process leads to coated metallic sheets with desirable properties, primarily to be used as interconnects in solid oxide fuel cells and solid oxide electrolysis cells.

1 . A process for conditioning the surface of a plate or sheet of stainless steel with a thickness of from 0.2 mm up to 8 mm and subsequently applying a layer, such as a ceramic or metallic layer, onto said conditioned surface by wash coating, screen printing, wet powder spraying, flame spraying or plasma spraying, said process comprising the following steps:
 a) optionally annealing the steel plate or sheet for up to 100 hrs in a protective gas atmosphere at a temperature of 600-1000° C. in order to segregate Si, Al, Ti and other oxidizable (electropositive) elements out in the surface,   b) controlled etching of the surface of the plate or sheet to produce a roughened surface with blind holes, i.e. closed or non-through holes, giving the surface a roughness Rz of between 3 μm and 50 μm and   c) depositing a protective and electrically conductive layer onto the roughened metallic surface, thereby forming a metallic oxide layer on the surface.   
     
     
         2 . The process according to  claim 1 , wherein the optional annealing in step (a) is conducted for 1 hr or more in a protective gas atmosphere selected from Ar and other inert gases, N 2  and H 2 . 
     
     
         3 . The process according to any one of  claims 1 - 2 , wherein the layer to be applied in step (c) is a ceramic or metallic layer. 
     
     
         4 . The process according to any one of  claims 1 - 3 , wherein the protective and electrically conductive layer is deposited onto the roughened metallic surface by thermal spraying, wash coating, screen printing, wet powder spraying, flame spraying, plasma spraying, PVD (physical vapour deposition), CVD (chemical vapour deposition) and galvanic processes. 
     
     
         5 . The process according to any one of  claims 1 - 4 , wherein the layer deposited in step (c) is composed of LSM (lanthanum strontium manganite), La—Sr—Cr—O, La—Ni—Fe—O, La—Sr—Co—O, Co—Mn—Ni—O or La—Sr—Fe—Co—O or consists of a perovskite material having the general formula ABO 3  or a spinel material having the general formula ABO 4  in which the elements A and B generally have oxidation states +2 and +3. 
     
     
         6 . The process according to any one of  claims 1 - 4 , wherein the coating applied in step (c) consists of Co or a combination of Co and Ni formed by PVD (physical vapour deposition), CVD (chemical vapour deposition) or a galvanic process. 
     
     
         7 . The process according to any one of  claims 1 - 4 , wherein the metallic layer is selected from high temperature oxidation resistant alloys. 
     
     
         8 . The process according to any one of  claims 1 - 7 , wherein the controlled etching in step (b) is carried out by using wet chemical or other etching methods. 
     
     
         9 . The process according to any one of  claims 1 - 8 , wherein the thermal spraying is a plasma spraying process carried out at a temperature where the coating powder is completely or predominantly melted. 
     
     
         10 . The process according to  claim 8 , wherein the etching is carried out by using a wet chemical method involving FeCl 3  and HCl. 
     
     
         11 . The process according to any one of  claim 8  or  10 , wherein the controlled etching is carried out by using a wet chemical method involving FeCl 3 , HCl, HNO 3 , NH 4 F or combinations thereof. 
     
     
         12 . The process according to any one of  claims 1 - 11 , wherein the etching is followed by oxidation in air at a temperature of 800-950° C. for 1-10 hrs before coating. 
     
     
         13 . The process according to any one of  claims 1 - 12 , wherein the stainless steel is a high-temperature ferritic stainless steel. 
     
     
         14 . The process according to  claim 13 , wherein the stainless steel is selected from Crofer® 22 H, Crofer® 22 APU, Sandvik Sanergy™ HT, ZMG 232L, ZMG J3 and ZMG G10. 
     
     
         15 . The process according to any one of  claims 1 - 14 , wherein the metal sheets prior to the etching are heat treated in a low O 2  containing atmosphere of H 2 , Ar or the like at a temperature of 600-1200° C. for 0-100 hrs in order to concentrate Si, Ti and Al close to or on the surface. 
     
     
         16 . A plate prepared by coating of a sheet of stainless steel using the process according to any one of  claims 1 - 15 . 
     
     
         17 . An interconnect plate (IC-plate) prepared by coating of a thin sheet of stainless steel using the process according to any one of  claims 1 - 15 . 
     
     
         18 . Use of the interconnect plate (IC-plate) according to  claim 17  in a solid oxide fuel cell (SOFC) stack or a solid oxide electrolysis cell (SOEC) stack. 
     
     
         19 . High temperature fuel cell stack comprising a plurality of interconnect plates (IC-plates) according to  claim 16 .